Field of the Invention
The present invention relates generally to antennas and feeds for dish antennas. In particular, this invention relates to ring focus dish antennas for use in communications systems. Still more particularly, this invention relates to an integrated antenna feed and a turnstile circular polarizer for use with a ring focus dish antenna.
Description of Related Art
High gain antennas, used in applications such as satellite communications (SATCOM), or long range line-of-sight (LOS) communications links, require large aperture areas to achieve sufficiently high gains. Two primary methods by which these large aperture areas can be achieved are through an array of small elements (array antenna) or through directing the RF energy to an antenna feed using a large area dish and a subreflector. The reflector may also focus directly to an antenna feed (primary feed reflector) instead of using a subreflector. The reflector can be fabricated in a plurality of ways to achieve the optics desired. Additionally, a large lens can be used to focus energy to an antenna feed.
In parabolic antennas such as satellite dishes, an antenna feed horn (or feedhorn) is a small horn antenna used to direct radio waves between a feedhorn, a subreflector, and a parabolic main reflector dish. The antenna can be transmit only, receive only (half duplex), or it can have both transmit and receive functionality, simultaneously (full duplex). In transmit mode, the feed horn is connected to the transmitter and converts the radio frequency energy from the transmitter to radio waves and feeds them to the rest of the antenna, which focuses them into a beam. In receiving mode, incoming radio waves are gathered and focused by the antenna's main reflector onto the feed horn, which converts the incoming radio waves into detectable radio frequency energy which may be amplified and further processed by the receiver. Transmission mode and receiving mode can occur simultaneously from the same antenna either through frequency division or through time division duplexing. Alternatively, transmission and receiving modes can occur individually.
Ideally, the aperture between the feed horn and subreflector of a ring focus reflector-type antenna is entirely unobstructed. However, in conventional reflector-type antennas, some form of mechanical structure is generally required to support the subreflector relative to the feed horn. However, such support structure, e.g., one or more struts, dielectric, etc., unavoidably shadows, attenuates, or blocks, a portion of the aperture between the feed horn and the subreflector and consequently degrades the performance of the antenna.
Another problem with a conventional antenna feed is that each of the components, e.g., input section, polarizer, feed horn and subreflector, is generally constructed as a separate component. The assembly, testing and fine tuning of such separately manufactured antenna feeds results in significant labor and manufacturing cost, long fabrication and test times, and potential for high variability of antenna performance between units.
Antennas located in space on a satellite are limited in material choices, and most dielectrics are not fit for space applications. Similarly, the use of struts degrades performance and increases the stowed size of the antenna, making it more difficult and expensive to launch.
Accordingly, there exists a need in the art for a high-gain antenna feed that alleviates at least some of these problems with conventional antenna feeds used with ring focus dish reflector-type antenna systems. For example, an antenna feed without dielectric or strut supports would be particularly useful in the SATCOM context.